The invention relates to a method for filling a gripper for bulk material.
It is generally known that grippers are used for handling bulk materials, such as e.g. ore, coal, grain, gravel or sand. These grippers which are also defined as a clamshell grab or grapple have a size, shape and number of shells optimized in each case with regard to the bulk material to be handled. This ensures that the grippers can penetrate into the bulk material in an effective manner, can be filled with the bulk material and the bulk material can be emptied therefrom in an effective manner. Typically, the grippers are lowered in an open position onto the bulk material, sink into the bulk material by reason of their own weight and during a closing movement the grippers pick up the bulk material and are filled therewith. The grippers are closed hydraulically or by means of cable drives.
A crane comprising a gripper for bulk material is known from German patent DE 199 55 750 B4. The gripper is designed as a so-called four-cable gripper. Accordingly, two holding cables and two closing cables are provided which can be moved independently of one another, in order to open, close, lift and lower the gripper. The holding and closing cables are driven separately by two cable drums. In order to open the gripper, the closing cables are relieved and the gripper hangs only on the holding cables. The holding cables act upon a lever mechanism of the gripper and serve, in conjunction with the weight of the gripper, to open the gripper. For a filling procedure, the opened gripper having a slack closing cable is placed onto the bulk material by means of the holding cables. In order to allow the gripper to sink into the bulk material under the effect of its own weight, the holding cable is then slackened. By tightening the closing cables, the gripper is then closed, wherein it is filled and subsequently raised by the closing cables after the gripper is closed. In this case, the holding cables must then be tautened in parallel, in order to avoid slack cable. In the region where the gripper is raised, the forces in the holding and closing cables are then adjusted with respect to one another via corresponding controllers, so that the subsequent lifting is effected jointly with the holding and closing cables.
Typically, data relating to the bulk weight density, gripper volume and the weight of the gripper are not input into a crane controller of the crane. Under the rough conditions of use of the gripper, a fill level of the gripper is taken into consideration only indirectly using empirical values.
A method for preventing overloading of a gripper suspended on holding cables and closing cables is already known from DD 288 138 A5. In this case, during the closing procedure a tensile force acting in the closing cables and in the holding cables is measured and its difference is compared with a target value for the tensile force acting in the closing cables. If the difference exceeds a specified value for the target closing force, the holding motor is activated, whereby the still not completely closed gripper is raised and continues to be closed. In this manner, the effect of the weight of the gripper, by means of which the gripper acts upon the bulk material during closing and filling, is reduced and a fill degree of the gripper is influenced.
Furthermore, a tension control for the gripper cables of a bulk material handling apparatus comprising holding cables and closing cables is known from EP 0 458 994 A1. In order to avoid a jerking movement which is caused by slack holding cables when the gripper is being closed, the holding tension is controlled accordingly. However, the holding tension is controlled in such a manner that during the closing procedure no reduced effect of the weight of the gripper on the bulk material is achieved because the gripper is only raised if it is completely closed. Therefore, the tension control does not influence the fill degree of the gripper.
DD 244 962 A1 describes a method for controlling pick-up of goods for an automated gripper operation. In this case, during the closing procedure of the gripper a gripper opening angle and a closing time of the gripper lowered onto the bulk material are monitored. If, during the closing procedure, the gripper cannot be closed to a specified extent within a specified time, the closing procedure is interrupted and a lifting gear is activated, in order to raise the gripper from the bulk material. Then, the closing procedure is restarted and a check is carried out to establish whether it can be performed as specified.
A method for filling a gripper suspended on holding cables is known from European patent document EP 2 226 287 B1, the gripper filling volume of said gripper being influenced in that a holding torque of a holding mechanism for the gripper is controlled in such a manner that during the closing procedure a gripping curve of the gripper is raised.